Generally, in a paper making machine, a wet paper web is formed in a former on a forming fabric and then carried by the forming fabric, or transferred therefrom to a drying fabric by which the web is then carried, through a drying section where it is at least partially dried. Water removal in the drying section may be accomplished through the use of one or more through-air dryers depending on the desired degree of drying of the paper web. Through-air drying typically works by passing a hot gas such as air, or a hot vapor such as steam, through the moist web using applied differential pressure. Water is then removed from the web by the drying medium principally by the mechanism of forced convection.
A rotary through-air dryer (TAD) is generally the most common type of through-air dryer used in paper making machines where non-compacting drying of the paper web is desired. A rotary TAD typically consists of a rotatable cylinder or drum with a porous face or deck. The porous deck is generally a welded honeycomb structure which gives the deck a high open area while providing the cylinder or drum with high structural strength. Further, since a paper web lacks strength, especially when wet, an air permeable backing or carrying fabric is typically required to support the paper web through the TAD. The fabric is configured to carry the paper web thereupon as it generally wraps about a major portion of the circumference of the cylinder. A number of fabric support members are also generally provided, both at the upstream side and the downstream side of the cylinder, to support and direct the fabric and the web onto and from the cylinder.
With the fabric and the paper web wrapped about the cylinder, the drying medium is then passed through both the fabric and the web to dry the web. For a rotary TAD, the hot air or steam drying medium supplied to dry the paper web may be directed to flow through the web and into the interior of the cylinder (an inward flow configuration) or may be directed out of the interior of the cylinder through the web (an outward flow configuration). Further, the drying medium may be directed through the deck twice in a cross-flow configuration, or may be conveyed between one or both ends or heads of the cylinder and the deck in an axial exhaust or axial supply configuration.
In order for rotary TADs to be effective in drying the paper web, the drying medium must be directed through the web and the carrying fabric as efficiently as possible. Thus, for the differential pressure to be applied to the rotary TAD to force the drying medium through the wet paper web and the fabric, the cylinder is enclosed to "duct" the drying medium from a supply to an exhaust. An enclosure for the cylinder typically includes a hood surrounding the portion of the cylinder about which the fabric and paper web are wrapped. The hood may be configured as a single-piece component or as a multi-piece assembly to facilitate serviceability. In some instances, a separate plenum may then be required to cover the "dead zone" of the deck over which the fabric and web are not wrapped. However, rotary through-air dryers are typically difficult to seal because of the need to pass the fabric and paper web into and out of the enclosure formed by the hood and, in some cases, a plenum. Inefficient sealing of the enclosure may lead to leakage, thus reducing the drying capacity of the dryer. Accordingly, several different methods have been devised for sealing a rotary TAD.
For example, U.S. Pat. No. 3,423,936 to Cole et al. discloses an inward flow rotary TAD arrangement wherein hot air is supplied through a hood covering the cylinder and the hood forms non-contacting seals with the drying fabric. The drying air flows through the paper web, the drying fabric(s), and through the deck of the cylinder into the interior thereof. The '936 patent discloses two methods for exhausting the air from the interior of the cylinder. As shown in FIG. 2 of the '936 patent, in a cross-flow exhaust arrangement, the air is exhausted through the dead zone of the cylinder and collected in an exhaust plenum disposed at that location. The plenum is arranged to form contact seals with the porous cylinder at the leading and trailing ends of the dead zone. The problem with attempting to form a seal with the porous cylinder is that a good seal may be difficult to achieve due to the expansive open areas in the deck of the cylinder and, accordingly, this design may tend to leak. Further, the seal at the leading edge of the plenum, with respect to the rotation of the cylinder, may have a tendency to dig into the cylinder and cause damage to the plenum and/or cylinder.
In addition, FIGS. 6 and 7 of the '936 patent disclose an alternate arrangement of a rotary TAD having an axial exhaust through one of the heads of the cylinder. However, the dead zone of the cylinder must still be sealed to prevent leakage of the drying medium therethrough. Accordingly, a stationary external shield is mounted over the dead zone with a slight clearance at the ends of the shield, between it and the outside surface of the rotating cylinder. The disadvantage of this configuration is that a non-contacting seal against a TAD deck is prone to leakage.
In another example, U.S. Pat. No. 3,303,576 to Sisson discloses an outward flow rotary TAD arrangement with a cross-flow configuration, using air as the drying medium. Heated air is flowed through a plenum covering the dead zone of the porous cylinder, through the deck and into the interior of the cylinder. The air then flows back out through the deck, the paper web, and the drying fabric(s) into a collection hood for exhausting the air from the TAD. According to the '576 patent, the supply plenum forms non-contacting seals at the interfaces of the deck and the fabric in the general location of the idlers, while the hood forms contacting trailing seals with corresponding idlers.
However, the rotary TAD configuration as disclosed by the '576 patent is limited to an outward flow configuration. An outward flow configuration in a rotary TAD is disadvantageous in that special provisions must be made in order to prevent the web from being damaged by the surface of the cylinder and the maximum pressure differential that can be applied (and thus the air flow per unit area) is limited by the maximum allowable fabric tension divided by the cylinder diameter. This relationship restricts the economic size of outward flow TAD cylinders since increasing the size of the cylinder results in a reduction in the allowable differential pressure and thus the air flow per unit area.
In a further example, U.S. Pat. No. 4,247,990 to Ohls et al. discloses an inward flow rotary TAD using air or steam as the drying medium. Both cross-flow and axial flow configurations are disclosed. The paper web and the drying fabric are directed onto and from the TAD cylinder between two or more sealing rolls disposed adjacent the cylinder. An enclosure is provided to seal the interior thereof from the outer atmosphere. Since the fabric and the web pass between the sealing rolls, the hood covering the portion of the cylinder about which the fabric is wrapped, as well as the plenum covering the dead zone, are sealed to the corresponding sealing rolls with sealing strips. However, passing the web and the fabric through a pair of adjacent rolls may have the effect of compacting the paper web and densifying the resulting paper sheet. Where the paper making machine is used to produce tissue and towel products, densification of the paper web is undesirable since it tends to reduce absorbency and softness of the paper product while increasing its stiffness.
Thus, it would be desirable to provide a rotary through-air dryer for a paper making machine capable of being configured in both a cross-flow and an axial flow configuration and capable of handling both inward flow and outward flow processes. It would be further desirable for such a through-air dryer to be effectively sealed. Effective sealing of the TAD would have benefits such as increasing the energy efficiency of the apparatus by minimizing leaks, allowing the use of smaller fans due to the reduced leakage, and permitting a greater pressure differential (and, consequently, a higher air flow per unit area) to be achieved. These benefits would further allow the apparatus to be reduced in size, thus resulting in a reduced capital cost. Such an improved TAD would further desirably provide an effective sealing system without undesirably compacting the paper web and without the risk of seals digging into the porous cylinder.